in geometry, a plane figure comprehended by a single curve line, called its circumference, to which right lines drawn from a point in the middle, called the centre, are equal to each other. See GEOMETRY.

Circles of the Sphere, are such as cut the mundane sphere, and have their periphery either on its moveable surface, or in another immoveable, conterminous, and equidistant surface. See SPHERE. Hence arise two kinds of circles, moveable and immoveable. The first, those whose peripheries are in the moveable surface, and which therefore revolve with its diurnal motion; as, the meridians, &c. The latter having their periphery in the immoveable surface, do not revolve; as the ecliptic, equator, and its parallels, &c. See GEOGRAPHY.

Circles of Altitude, otherwise called almucantars, are circles parallel to the horizon, having their common pole in the zenith, and still diminishing as they approach the zenith. See ALMUCANTAR.

Diurnal Circles, are immoveable circles, supposed to be described by the seven stars, and other points of the heavens, in their diurnal rotation round the earth; or rather, in the rotation of the earth round its axis. The diurnal circles are all unequal: the equator is the biggest.

Hourly Circles, in dialing, are the lines which show the hours on dials; though these be not drawn circular, but nearly straight. See DIALING.

Circles of Latitude, or Secondaries of the Ecliptic, are great circles parallel to the plane of the ecliptic, passing through the poles thereof, and through every star and planet. They are so called, because they serve to measure the latitude of the stars, which is nothing but an arch of one of these circles intercepted between the star and the ecliptic. See LATITUDE.

Circles of Longitude, are several lesser circles, parallel to the ecliptic; still diminishing, in proportion as they recede from it. On the arches of these circles, the longitude of the stars is reckoned.

Circle of perpetual Apparition, one of the lesser circles, parallel to the equator; described by any point of the sphere touching the northern point of the horizon; and carried about with the diurnal motion. All the stars included within this circle never set, but are ever visible above the horizon.

Circle of perpetual Occultation, is another circle at a like distance from the equator; and contains all those stars which never appear in our hemisphere. The stars situate between these circles, alternately rise and set at certain times.

Polar Circles, are immoveable circles, parallel to the equator, and at a distance from the poles equal to the greatest declination of the ecliptic. That next the northern pole is called the Arctic; and that next to the southern one the Antarctic.

Fairy Circle. See Fairy-Circle.

logic, or Logical Circle, is when the same terms are proved in orbem by the same terms; and the parts of the syllogism alternately by each other, both directly and indirectly.

Circles of the Empire, such provinces and principalities of the German empire as have a right to be present at diets. Maximilian I. divided the empire into six, and some years after into ten circles. This last division was confirmed by Charles V. The circles, as they stand in the Imperial Matricola, are as follow: Austria, Burgundy, the Lower Rhine, Bavaria, Upper Saxony, Franconia, Swabia, Upper Rhine, Westphalia, and the Lower Saxony.